The present invention generally relates to devices and methods for switching electric signals, as they may be used, for example, in integrated circuits.
In integrated and discretely set-up circuits of digital technology and power electronics, mainly transistors based on metal-oxide semiconductor field-effect transistor (MOSFET) or metal-insulator semiconductor field-effect transistor (MISFET) technology are used as electric switching devices, as they show a comparatively low drive overhead. Complementary designs, n or, respectively, p channel transistors of a normally off (enhancement transistor type) or normally on (depletion transistor) transistor type are used.
Transistors which are based on MOS or MIS technology, respectively, represent the main portion of electric semiconductor switching devices. The MOS or MIS technology, respectively, is continually being improved in the field of applied materials. For example, a silicon substrate may be used with an epitaxially grown silicon-germanium layer, whereby conductivity in the area of the inversion channel is increased due to the higher mobility of the charge carriers. Depending on the application, metals, polysilicon or silicates are used as electrode materials for the gate, source or drain contacts, respectively. In the course of constant miniaturization of the dimensions of the MOS structures, also for the gate dielectric instead of SiO2 alternative layer materials are used, e.g. high-epsilon layers like hafnium dioxide or aluminum dioxide.
Further improvements of the MOS or MIS technology, respectively, refer to geometric aspects of the MOS structure. For example, for this purpose the process of lightly doped drain (LDD) contact may be used for preventing electric field peaks close to the drain area or the FinFET technology for an improved implementation of an inversion channel (keyword: “double gate”).
A further improvement of the MOS or MIS technology, respectively, with regard to the increase of the charge carrier mobility may be achieved by a strained silicon. Strained silicon consists of a silicon-germanium layer (SiGe) onto which a thin silicon layer is applied. Due to the fact that the SiGe layer has a crystal structure with a higher lattice constant, i.e. with greater distances between the individual atoms, at the contact location of the SiGe and the Si layers, the crystal lattice of the silicon is somewhat expanded, so that also the distances between the Si atoms become larger. Due to the greater distances between the atoms, the electric conductivity or the charge carrier mobility is increased, respectively. This in turn leads to a faster transit of the electrons through the silicon layer and thus allows a faster switching speed of a transistor and thus also a faster clocking of a processor. By the respective change or deformation of the crystal lattice in the area of the channel area, i.e. by a targeted introduction of germanium atoms into the silicon substrate and/or by depositing a pressure- or tension-generating nitride compound above the gate contact thus the effective mass or, respectively, the mobility of the charge carriers in the inversion channel of a transistor may be affected. These changes or deformations of the crystal lattice structure, respectively, as are caused by the “strained silicon” technology, are unique and permanent interventions into a semiconductor crystal.
In principle, the function of an MOS structure is still applied for switch control or drive, i.e. an electric field (across the applied gate voltage) is further necessitated to generate an inversion channel at the boundary surface (interface) of the substrate and the gate dielectric.
Basic requirements with regard to an electric switching device are a low series resistance in the ON (conducting) operation and a low leakage current in the OFF (non-conducting) operation. Additionally, in many cases of application, e.g. in digital data processing or high-frequency technology, fast switching cycles between the ON and OFF state of the electric switch are necessitated. MOS-controlled switches may be adapted to the requirement profile via design parameters like geometric dimension, doping, etc., generally, however, a trade-off between the on or off characteristics of the transistor, respectively, has to be found, e.g. the DMOS transistor (double diffused MOS).
A further disadvantage of MOSFET or MISFET technology, respectively, is that only one charge carrier type (the majority charge carrier) contributes to the current flow. This limitation is not given with bipolar transistors. Here, both the majority and also the minority charge carriers contribute to the current flow. This leads to especially high current densities in the case of passage. Bipolar transistors, however, generally necessitate a costly control and have a relatively high leakage current in the OFF state. Thus, they are not suitable for many applications, in particular with regard to current-saving electronics, e.g. in mobile devices.